It was a great bike in it's day. I haven't ridden it in years. I let my sis-in-law use it. She let it
fall into disrepair. Now I want to start riding again, but I need to, well want to really, upgrade the bike
from an 8 spd to a 9 spd bike.

I'm basically plan on leaving the HS, fork, stem, bars, seatpost, brake levers, and wheels alone. That leaves just about everything else open to adjustment.

The bike was built up with an XT gruppo (you can see I've spent most of my time on the road), with the exception of Onza brakes. Is it possible to keep the rear (8spd) derailluer alone and upgrade the rest of the drivetrain?

What tires are the best for XC on loose, rocky terrain? I live in the Salt Lake area if that helps.